Inexpensive pressure and temperature gauges are commonly made using spring elements known as Bourdon tubes. A thin-walled metal tube is flattened along its length, but is not completely closed on its inside when so flattened; the flattened tube is formed into a curve and sealed at one end; and the inside of the tube is subjected to a source of pressure applied through its other end. The applied pressure causes the tube to undergo a volumetric change, deflecting the sealed end of the tube a measured amount.
One problem with such springs is that the applied pressure often causes the thin metal walls to split, especially along the sharply folded edges of the flattened tube where the stresses are concentrated. Caldwell U.S. Pat. No. 2,495,314 eliminates the sharply folded edges by positioning a round wire form along each inner edge of the tube, and flattening the tube around the wire to form round edges having relatively large radii of curvature. To further prevent stress concentration at the edges of the tube, Caldwell also shapes his rounded edges into sections having transversely varying wall thickness to provide a theoretically uniform transverse stress distribution. However, Caldwell's complexly shaped spring with its cumbersome wire forms is difficult and costly to fabricate.
Another common problem with Bourdon springs is that the tube permanently deforms under the applied pressure, rendering the gauge inoperative. The problem is particularly acute where the gauge is subjected to cyclical pressure loading, because the thin-walled metal tubes are especially prone to fatigue. One approach to this problem has been to fabricate the tube from a material that can be heat treated to a spring temper, such as beryllium copper alloy. However, there are many very important applications for which such copper alloy springs are useless, because the metal reacts with the pressurized fluid causing the tube to develop leaks (e.g., acetylene torches, fire extinguishers with antifreeze additives) or causing unacceptable contamination (e.g., oxygen cylinders for medical use). For such applications especially, it would be desirable to fabricate the spring from a relatively inert metal, such as stainless steel. However, attempts to produce an inexpensive stainless steel gauge with adequate strength and spring temper have not been successful, because stainless steel is soft and cannot be heat treated to a spring temper.
Another approach to this problem has been to make a gauge with a short spring which has a very small throw or movement when loaded. Short springs are less prone to fatigue, because of the low magnitude of the movement involved. However, complex and costly gearing mechanisms are required to amplify the small movements into readable output.
Perkins U.S. Pat. No. 3,232,116 shows a spring, made from material such as stainless steel, which is provided with welded strengthening ribs to reduce wall stresses. However, such welding is time consuming and expensive.
It would be desirable to provide a pressure responsive spring element having adequate spring temper and resistance to splitting which does not require heat treating or costly fabrication procedures.